A cease-fire between Israel and Hamas took hold across the Gaza Strip border on Thursday, the day after Egypt announced a halt to eight days of Israeli-Palestinian bombardments, the worst fighting in four years. But despite the initial calm, it was still unclear how long the cease-fire would hold up, with both sides warning they were ready to renew hostilities if necessary.

Israelis and Palestinians awoke to a morning without explosions and sirens and came out of their hiding places, in contrast to the rain of Palestinian...